There is increasing evidence that adhesion molecules play a critical role in regulating numerous biological and pathological events including cell differentiation. However, the events and precise mechanism(s) by which adhesion molecules control cell differentiation remain to be determined. The studies proposed here focus on determining the role of the adhesion molecule, osteopontin (OPN), during cell differentiation using HL-60 cells. OPN, a bone-associated adhesion molecule, has been suggested to play a role in mediating osteoclast attachment to mineralized matrices. HL-6O cells, a human promyleocytic leukemic cell line, are of interest because they normally grow in suspension, but when exposed to such factors as retinoic acid or calcitriol differentiate into granulocytes or monocytes. Furthermore, as they differentiate they adhere and spread on culture dishes. Importantly, we have shown that one result of calcitriol differentiation is increased production of OPN. The hypothesis upon which this study is based is calcitriol's (1,25 dihydroxyvitamin D3) ability to induce differentiation of HL-60 cells into monocytes is related to its known ability to increase the levels of protein kinase C (PKC) and ultimately related to the production of OPN. PKC has been shown to enhance cell adhesion by increasing cell surface adhesion receptors., Specifically, the studies detailed in this proposal are directed at determining if inducers of differentiation regulate OPN expression by a mechanism involving PKC and if this process is essential for terminal differentiation of HL-60 cells. The major objectives of these studies are to l) establish the role of PKC activity in calcitriol regulation of OPN gene expression and 2) determine if OPN associated integrins are important for calcitriol mediated differentiation of HL-60 cells to monocytes. The information obtained from these studies will enhance our understanding of the factors/mechanisms regulating development, maintenance and repair of tissues. Moreover, these studies will further define the role of extracellular matrix in induction and path selection of progenitor cell transformation. Long term such information may prove to be clinically useful in designing regenerative protocols and in controlling cell transformation.